This invention relates to the field of trunked communications systems having a base station and a multiplicity of remote units and, more particularly, to a means for providing positive identification of every unit using the system.
Trunked communication systems are systems wherein a multiplicity of units or subscribers have access to the same group of channels as needed. Telephone systems are an example of trunked systems and so are most two-way radio communication systems where the number of available channels is limited, full-time use of a channel would be more costly, and no one user needs full-time access to a channel. Trunked systems have often been termed either "message trunked" systems or "transmission trunked" systems. In both of these types, a dual control channel is used along with a number of voice channels. Each mobile unit has a unique identification code (ID) which includes its own individual code and its permanent group code. Using the control channel and its complete ID code, the mobile unit requests the use of a voice channel and, if a channel is available, a "channel grant" is given via the control channel. The mobile unit then switches to the assigned voice channel and communicates as desired.
In message trunking a timer in the control unit provides a time-out-time (TOT) of perhaps five or six seconds, during which the message or conversation can be continued on that same channel by again depressing the PTT. When the original mobile unit "dekeys" or releases the PTT, the channel remains available during the TOT period during which that unit or another unit can transmit on that channel without requesting it via the control channel. Thus, a situation is created whereby an unidentified unit could be using a voice channel if the channel was accessed soon after an identified transmission ended. Also, the five or six seconds of unused channel time was a significant, inefficient use of the channel.
In public safety operations (police, fire, etc.) it is highly desirable to have each transmitting unit identified, both in real time, and by recording of all voice channels for later identification of "who said what, and when". "Transmission trunking" is one solution to the identification problem. In transmission trunked systems there is no time-out-time and a request to transmit (including an ID) must be sent on the control channel before any mobile unit can access a voice channel. This arrangement solves the problem of unidentified units and is more efficient of channel time, but is not practical when used with systems including so-called wireline control consoles; i.e., systems wherein a dispatcher has a console with direct coupling between his microphone/speaker and the transmitter/receiver, respectively, of the assigned voice channel repeater. The difficulty arises from the fact that the dispatcher has priority while talking and the mobile units are required to listen while he is transmitting. In other words, the mobiles could not access the control channel, and thus could not use the voice channels, until the dispatcher dekeyed. Also, no other mobile unit can use that channel until the first unit has completed its conversation or data transmission and has released the push-to-talk button (PTT).
Another approach which has evolved, termed "quasi-transmission trunking", provides for a very short (0.5 to 1.0 sec) TOT during which a channel remains assigned, reducing the wasted channel time considerably but still allowing any unidentified unit to access the assigned channel during the TOT.
A typical trunked "system" may consist of a large number of mobile units comprising several "fleets". In public safety applications, one fleet might be a police department or a portion thereof, another fleet might be a fire department while other fleets might be mobile medical units, etc. Within a fleet there would be working groups or "subfleets" such as the mobile units of one police district. It is apparent, then, that a subfleet would mostly communicate within its own group, but could also need to communicate with another subfleet, even from another fleet. For example, at a large fire, a fire department subfleet would need communication, not only with other groups of firemen within its fleet, but also with the police units at the scene. The dispatcher must have the capability of communicating with whatever group or groups are necessary in a given situation. Such communications between different groups or "subfleets" in a trunked system may be termed temporary console "patches" in analogy to the former use of "patch cords" on telephone switchboards or "dynamic regrouping". This capability has been disclosed in a co-pending patent application, Ser. No. 770037. In the case of such regroupings or "patches", however, it is still important to be able to identify each unit which transmits.